Sidecar essentially replicates the functionality of a physical Luna Displaydongle that AstroHQ sells for $79.99—a seemingly steep price considering that Sidecar is free (see “Catalina’s Sidecar Turns an iPad into a Second Mac Monitor,” 21 October 2019, and “Luna Display Turns an iPad into a Responsive Mac Screen,” 7 December 2018).
So, what was AstroHQ to do? Company co-founder Matt Ronge took to Twitter earlier this month to announce Mac-to-Mac Mode for Luna Display.
If such a Mac-as-secondary-display set-up sounds vaguely familiar, you might be thinking of Apple’s own Target Display Mode, which does pretty much the same thing, but only under limited circumstances. You can use it to turn certain older iMacs into second displays for other Macs. But these setups tend to be temperamental.
TidBITS publisher Adam Engst used an older 21.5-inch iMac as a secondary display with his MacBook Air for a while, but the experience wasn’t entirely satisfying. He found that he often had to reinvoke Target Display Mode after the MacBook Air woke up, and he sometimes had to restart either the MacBook Air or the iMac to get it to work. Plus, it required keeping a keyboard attached to the iMac—purely to press Command-F2 to enable Target Display Mode—and while the iMac was hooked up, it somehow prevented audio from playing on the MacBook Air.
AstroHQ thinks the limited hardware requirements of Target Display Mode and its general flakiness give the Luna alternative an opening (shall we call it “Watsoning”?). For one thing, AstroHQ’s solution supports a wider range of dual-Mac scenarios—side-by-side MacBooks, for instance, or even a Mac mini with a MacBook functioning as its sole display.
But Mac-to-Mac Mode isn’t without its own issues. Most notably, although Mac-to-Mac Mode works with side-by-side iMacs, it doesn’t support either 21.5-inch or 27-inch iMacs with Retina displays—either 4K or 5K. That’s a shame, since it means you can’t use an older 27-inch iMac with Retina display as a 5K external monitor.
That said, Mac-to-Mac Mode does support standard 4K displays.
Getting Started with Mac-to-Mac Mode
In addition to one of the dongles, Mac-to-Mac Mode set-ups require the following:
- A “primary” or host Mac, from which screen content will be projected to a secondary Mac. You plug the Luna dongle into this Mac, which must be running OS X 10.11 El Capitan or later.
- The secondary Mac, which must be running 10.8 Mountain Lion or later.
- Two Luna apps. You run the Luna Display app on the primary Mac and the Luna Secondary app on the secondary Mac. Both apps are free when you buy the Luna Display dongle.
- A reliable network connection. Most people will typically use Mac-to-Mac Mode over Wi-Fi with both Macs on the same wireless network. It also works over Ethernet, if both Macs are connected to the same Ethernet network.
Enabling a Mac-to-Mac connection is straightforward, as it tends to be with a Luna-initiated Mac-to-iPad connection.
First, launch the Luna Display app on your primary Mac. If you’re running 10.14 Mojave or later, you might be asked to modify Accessibility settings in System Preferences, but you should need to do this only once.
Second, open the Luna Secondary app on your secondary Mac. At this point, the two Macs should be able to link up automatically. After a bit of display flashing, your Desktop should expand from the primary Mac to the secondary Mac.
Automatic connections worked fine for me on some attempts, but not others for reasons I couldn’t discern.
If an automatic connection does not work, you can employ a workaround called Wi-Fi Manual Connect. Click this button in the apps on both Macs. You should then see a code appear in the app on the primary Mac. The same code also will show up in the app on the secondary Mac, but with a few digits omitted—fill those in, and you should be good to go.
Wi-Fi Manual Connect worked well for me when automatic connections failed.
Once connected, you can use the Luna Display app on the primary Mac to tweak how the two Macs function in relation to each other, with the secondary Mac on the left or right. Clicking Custom dumps you into the standard Display settings for additional adjustments.
Using the Mac-to-Mac Mode
To test Mac-to-Mac Mode, I persuaded my wife to part temporarily with her trusty 2015 MacBook Air, which I paired with my 2018 Mac mini (see “It Lives! Apple Announces Pro-Focused Mac mini,” 30 October 2018) and its LG UltraFine external display (see “Apple Debuts LG’s All-New 23.7-inch UltraFine Display,” 20 May 2019).
With the MacBook Air in hand, I tested Mac-to-Mac Mode in three scenarios. Two of these involved the USB-C Luna Display dongle, and the third tapped the Mini DisplayPort version.
1: MacBook Air As Secondary Display
My goal here was to use my LG UltraFine display as the main screen for my Mac mini, with the MacBook Air off to one side as a secondary monitor, much as I would when using an iPad with the Luna Display.
This dual-screen setup worked well, with my Mac mini’s extended Desktop displaying on the MacBook Air with its full 1400-by-900-pixel resolution, and with good responsiveness.
The Mac-to-Mac Mode connection was impressively hard to break. I repeatedly closed and reopened the MacBook Air’s lid, with the Luna Display hookup continuing as if nothing had occurred. Putting the Mac mini to sleep did break the Mac-to-Mac Mode link, as you’d expect, but the Luna apps were waiting upon reawakening and automatically reinitiated the connection.
I did notice one bizarre anomaly. While both the MacBook Air’s built-in keyboard and the Mac mini’s external keyboard behaved as expected for text input, only the Mac mini’s mouse and trackpad acted normally. When using the MacBook Air’s trackpad, I couldn’t move the cursor onto the Mac mini’s display—It was notebook-only, for some reason.
2: MacBook Air As Primary Display
What if you have a Mac mini as a “headless” Mac—tucked away in a closet, perhaps? Luna Display’s Mac-to-Mac Mode lets you use a Mac laptop as a wirelessly connected monitor whenever you need it. AstroHQ has already proposed using an iPad with a Mac mini in this way.
In my case, Mac-to-Mac Mode on my wife’s MacBook Air enabled me to use my Mac mini while on the comfy sofa far from my office.
To test this single-display solution, I had to physically disconnect my LG screen from the Mac mini so that the MacBook Air would become the Mac mini’s sole display.
But first, a few macOS tweaks were required for this to function properly:
- Automatic login must be turned on via System Preferences > Users and Groups > Login Options. A pull-down menu at the top lets you designate a Mac account for automatic login. If the pulldown menu is grayed out, check if FileVault is turned on (it must be off) or if an iCloud password is used for authentication (which also disables automatic login).
- In that same Mac account, the Luna Display app must be set up to launch automatically on login (in System Preferences > Users and Groups > Login Items).
Once I had twiddled those settings, I rebooted my Mac mini while launching the Luna Secondary app on the MacBook Air. A few seconds later, the Luna apps on the Macs found each other, and—presto!—I was looking at the Mac mini desktop on the MacBook Air’s screen.
Everything worked reasonably well. Responsiveness was a bit slow but not bad. All input devices—the Mac mini’s Magic Mouse, Magic Trackpad, and Magic Keyboard, along with the MacBook Air’s built-in laptop keyboard and trackpad—behaved as expected. Dependability was decent; closing and reopening the MacBook Air’s lid had no effect on the Mac-to-Mac Mode link.
Because the MacBook Air and Mac mini connected over Wi-Fi, I was able to work anywhere within the range of my Wi-Fi network. In fact, I wrote much of this article on the MacBook Air in my first-floor living room, wrapped in a warm blanket on the sofa (because winter is nigh here in Minnesota), with the Mac mini running on the second floor in my home office. Sweet.
3: MacBook Air As Primary Computer
Until now, my Mac mini has served as the primary computer in my Mac-to-Mac Mode experiments, with my wife’s MacBook Air as a secondary display. But what if I flipped things around and made the MacBook Air my primary computer, with my Mac mini’s LG monitor serving as a secondary display for the MacBook Air?
That’s where the Mini DisplayPort version of the Luna Display dongle comes in. I plugged it into the MacBook Air for use with the primary Luna Display app, and then I ran the Luna Secondary app on the Mac mini. The goal was to extend the MacBook Air’s Desktop onto the LG display, via the Mac mini. At least, that was the idea.
Unfortunately, I could not get this setup to work reliably. The Mac-to-Mac Mode connection crashed repeatedly. At times I could move the cursor between displays, but at other times, it refused to move.
It was a mess, as AstroHQ’s Matt Ronge acknowledged when I got in touch. He said his team was scrambling to fix bugs that interfered with the use of the Mini DisplayPort model of the Luna for Mac-to-Mac Mode connections. I plan to check back on this and leave any updates in a comment below.
For now, though, although I can recommend the USB-C version of the Luna Display dongle with Mac-to-Mac Mode, if your main Mac only has a Mini DisplayPort or Thunderbolt 2 port, hold off for a bit.
Revisiting Luna Mac-to-iPad Mode
Dusting off my Luna dongles, months after writing about how they enable iPads as secondary Mac monitors, allowed me to revisit that scenario — and compare it to Apple’s equivalent, the Catalina Sidecar feature I reviewed just last month.
Because of Sidecar’s oddities, the Luna approach came as a relief in some ways. Of particular note: Using macOS with touch on an iPad screen is better with AstroHQ’s implementation. You can reach out and do just about anything with a fingertip — which seems sort of obvious, right? Ah, but as I detailed in my Sidecar review, fingertip interaction will work for some things on the iPad screen, and not others. In many cases, an Apple Pencil is required. Ugh.
So, for those who can afford the purchase of the Luna hardware, this is the best way to turn an iPad into a Mac display. Because Sidecar is free, though, most people will end up using that — which is fine since it works reasonably well, the touch issues aside, and should get better over time.
Mac-to-Mac Mode: Bottom Line
Using one Mac as a secondary display for another Mac will not have occurred to most users because, until now, this has been possible only under narrow circumstances involving specific iMac models. And, as Adam Engst notes, Apple’s Target Display Mode doesn’t work very well.
AstroHQ, scrambling to remain relevant after being Sherlocked, has therefore done the Mac world a huge favor with its Mac-to-Mac Mode. Though not yet fully baked, especially on the DisplayPort end, Luna hardware and its related software are a godsend for those with extra Macs ready to be repurposed as second displays.
This, of course, comes at a cost—the Luna dongles don’t come cheap, so every prospective user will have to weigh whether this is money well spent.